1. Field of the Invention
The present invention relates to a double-shielded electroluminescent panel, more particularly to an electroluminescent panel against current leakage and electromagnetic interference.
2. Description of the Related Art
The great progress in electroluminescent device manufacturing has led to a rather large increase in the brightness of an electroluminescent device. In this regard, the electroluminescent device is capable of acting as the backlight source of a large-scale outdoor signboard such as a signboard with an area of from 60×90 cm2 to 100×140 cm2. Generally speaking, a user can control the operation voltage and alternating frequency of the electroluminescent device to change its brightness, wherein the range of the operation voltage is from 6V to 220V and the range of the alternating frequency is from 50 Hz to 1,500 Hz. The adjustment of the operation voltage or the alternating frequency controls the brightness or hue of luminescence. 140 V accompanied with 1,200 Hz is a currently popular driving specification for the driving power source of the electroluminescent device.
FIG. 1 is a schematic cross-sectional diagram of a conventional electroluminescent device. The electroluminescent device comprises an upper insulation layer 11, a front electrode layer 12, a fluorescence layer 13, a dielectric layer 14, a back electrode layer 15 and a lower insulation layer 16. The fluorescence layer 13 emits fluorescent rays through the upper insulation layer 11 after being excited by electrical energy. Therefore, the upper insulation layer 11 is the illuminating surface of the electroluminescent device 10. The front electrode layer 12 is made from a transparent ITO (Indium Tin Oxide) material, and the back electrode layer 15 is formed by coating or printing silver or carbon paste on the dielectric layer 14.
The electroluminescent device 10 applied to a large-scale signboard is generally fixed to the surface a metal plate or a metal frame. The metal plate is erected at an arresting place for public display. Because the environment moisture becomes higher or the dielectric coefficient of the lower insulation layer 16 is large enough, stray capacitors exist between the electroluminescent device 10 and the metal plate and result in current leakage.
FIG. 2 is an explanatory diagram illustrating current leakage and an electric shock occurring in the application of a conventional electroluminescent device. The electroluminescent device 10 is fixed to a metal plate 21, supplied with electrical power from an electrical source, and connected to an indoor socket 80 through an inverter 22. The common socket 80 includes three insertion holes respectively of a live line 81, a neutral line 82 and a ground line 83. In comparison with the socket 80, the inverter 22 has two plug terminals 221 and 222 connected to the neutral line 82 and live line 81, respectively. Because stray capacitors 231 exist between the electroluminescent device 10 and metal plate 21, electric charges accumulate on the surface of the metal plate 21. When a voltmeter is used to measure the voltage between the metal plate 21 and ground line 83, a considerable voltage difference exists between them. When a person touches the metal plate 21, he gets an electric shock caused by an electric current through his body. If the area of the electroluminescent device 10 is over 1,000 cm2, the driving source of it is set to the specification of 140V and 1,200 Hz so the voltage between the metal 21 and ground line 83 is higher than 1,000V. Meanwhile, a stray capacitor 231 exists in the inverter 22, hence the person touching the metal plate 21 would be a part of the circuit loop.
The structure of the conventional electroluminescent device 10 is too simple to be free from the danger of an electric shock. U.S. Pat. Nos. 5,899,549 and 6,528,941 respectively disclose an electroluminescent device with a lower shield layer that protects the components attached to the backside of the electroluminescent device from electromagnetic interference.
The electroluminescent device is a planar light source, and can display a large-scale image. When the specification of the driving source is 140V and 1,200 Hz, numerous electromagnetic waves are radiated from the illuminating surface. In this regard, the operation environment of reduced electromagnetic radiation does not comply with this fact.
In summary, an electroluminescent combination is in an urgent need of avoiding current leakage for the electroluminescence market to overcome the aforesaid problems.